1. Field of the Invention
This invention relates to an air-fuel control system for use with an engine carburettor and, more particularly, to an air-fuel control system responsive to various engine operating factors for controlling the air-fuel ratio of a mixture supplied to the engine according to engine operating conditions.
2. Description of the Prior Art
It has been considered preferable to maintain the air-fuel ratio of a mixture supplied to an internal combustion engine at the stoichiometric level. In terms of higher engine operating stability, however, the air-fuel ratio should be controlled in accordance with the condition under which the engine is operating. For this purpose, carburettors have been required to have the following functions:
During engine starting operation, the requisite functions are (1) to supply a rich mixture to permit the engine to start even at 30.degree. C., (2) to rarefy the mixture to a suitable fuel density after perfect explosion, (3) to permit the engine to run at high speed so as to prevent the engine from stalling after the engine starts, and (4) to gradually rarefy the mixture as the engine is warmed up. The function (1) has been accomplished by means of a choke valve and a valve operating bimetal, the function (2) by means of a perfect explosion diaphragm mechanism, the function (3) by means of a fast idle mechanism, and the function (4) by means of a heater for heating the choke valve operating bimetal.
During engine warm-up operation, the requisite functions are (1) to gradually reduce the speed of rotation of the engine to an idling level as the engine is warmed up, (2) to enrich the mixture if the engine is rapidly accelerated, (3) to permit high drivability even if the vehicle runs just after engine starting operation, and (4) to open the choke valve after the engine is warmed up for fuel economy. The function (1) has been accomplished by means of an unloader mechanism, the function (2) by means of a perfect explosion diaphragm mechanism and an acceleration pump, the function (3) by means of all of the above mechanisms, and the function (4) by means of a heater for heating the choke valve operating bimetal.
After engine warm-up operation, the requisite functions are (1) to supply a mixture of a constant fuel density regardless of throttle opening changes, (2) to supply a mixture of a suitable fuel density if the engine is accelerated or decelerated, and (3) to supply a power mixture if the engine is under high loads. The function, (1) has been accomplished by means of an oxygen sensor for feedback control of the air-fuel ratio, the function (2) by means of an acceleration pump and a deceleration device, and the function (3) by means of a power mixture supply mechanism.
It can be seen from the foregoing that various means have been used to control the air-fuel ratio in accordance with engine operation. This results in a complex and expensive carburettor.
Electronic controlled carburettors have been proposed in the art which comprise an oxygen sensor such as, for example, a zirconia oxygen sensor provided in the exhaust system of the engine for detecting the oxygen density of the exhaust gases discharged over it to derive the fuel-air ratio of the mixture supplied to the engine for feedback control of the air-fuel ratio. However, such conventional electronic controlled carburettors have been intended to maintain the air-fuel ratio at a predetermined level regardless of engine operating condition.